Mathematical Ratio among three point lights
There is no such formula, as lighting depends on the intentions of the artist. 

Moreover thinking in three point lighting is also quite limiting, Sometimes you just need one light, sometimes you need more than 10 sources...

Some questions to ask yourself are:

What are the characteristics of the object that you want to portray? How is your lighting going to enhance, reveal or suppress them? Where is the main light source coming from? Do you want defined shadows or do you want soft ones? How dark do you need the shadows to be? How is the shadow of an object going to affect others in the scene or how are the shadows of other objects affecting the one you are focusing on? Can the lights affect how you perceive the shape of objects? Should your object be brighter or darker than the background? How would the light help guide the viewer on reading an image? How are the bright and dark areas affecting the overall composition of the image?

There are endless variables, and the history of art going back to cave paintings deal with solutions to particular challenges... Study images you like and think of where the light is coming from and how the the different sources interact.  

Try things, experiment with placement, size, intensity and color and you'll find answers according to your taste.